1. Field of the Invention
The invention relates to a method and an arrangement for magnetic digital recording with high frequency biasing.
2. Description of the Prior Art
The magnetic recording of a data signal consists basically in creating a write current in a magnetic head, which produces on one face of a magnetic support such as a tape, disc or drum a residual magnetisation representative of the initial data.
In the case of an electric analog data signal, for example a signal whose amplitude is representative of sound from an acoustic transducer, it is evident that the residual magnetisation created by the write current should faithfully respect, in time, the amplitude of the data signal. This is universally obtained by superimposing on the data signal a high frequency, constant amplitude, alternating current signal which constitutes a magnetic biasing. Put briefly, the curve of first magnetisation of the magnetic material of the recording carrier starts with a quadratically curving part, extends in a linear part and ends in a part which curves in to end in saturation. Without the magnetic biasing current, the recording of the data signal would correspond to making a more or less large excursion in the curve of the first magnetisation from its origin and at each point of the recording carrier according to the level of amplitude of the write signal. The quadratically increasing part of the curve would always be involved so that the recorded signal would present a high rate of distortion. The superposition on the data signal of a high frequency, constant amplitude, alternating current signal constitutes a magnetic biasing in the sense that the excursions can be made in the linear part of the curve of the first magnetisation. Furthermore, the high frequency magnetic biasing leaves the recording carrier non-magnetised in the absence of the data signal, the more so since the length of the corresponding wave of the biasing current is inferior to the power of resolution of the read head, which power is essentially dependent on the value of the read air gap. On the other hand, although a direct current magnetic biasing is also possible, it would place the recorded carrier, in the absence of the data signal, in a permanently magnetised state which would have repercussions in the read signal in a great background noise.
A binary coded digital data signal indicates successively, at a given recurrence frequency, the value 0 or 1 of a data bit. This signal comprises, therefore, two correlative components--a repeated series of instants and a series of corresponding binary values--which the residual magnetisation should normally translate faithfully. To do this, it has been attempted to represent at least one of the two binary values by a magnetic flux transmission determined as a function of a selected code occurring at a precise corresponding instant.
In current practice, magnetic transitions are advantageously inversions of biasing of the residual field, designed to make this field change between two predetermined positive and negative biasing levels of the magnetic material of the recording carrier. The result of this is to create in this carrier a set of magnets placed end-to-end, with adjacent poles of the same kind, and of length corresponding to the time interval separating two transitions conforming to the method of coding chosen. By convention, a reversal of the residual field from a negative level to a positive level of biasing will be called a positive transition, a reversal in the opposite direction being a negative transition.
Among the types of coding most used is that called NRZ1 (non return to zero for bits of value "1"). In the NRZ1 code, only bits with a value of 1 are represented by magnetic transitions independently of the direction of these transitions. In another popular code called "coded phase," wherein the two binary values correspond respectively to the positive and negative transitions. As will be seen later, the method of coding chosen does not matter for the purpose of the present invention.
Various problems are, in effect, related to the accuracy of recording and reading of the other component of the digital data signal relative to the instants at which the transitions should have taken place.
It has been noted previously that the binary data is translated on the recording carrier as a series of magnets placed end-to-end, of which the adjacent poles are of the same kind and translate the existence of a transition. The read current produced by the read head during passage of two adjacent semi-magnets is, therefore, in the form of a clock curve, of which the peak corresponds to the transition, since the variation in magnetic flux in the read winding is greatest during passage of the two neighbouring poles of the two magnets before the air gap of the read head. However, when two transitions are very close together (which is the case with high recording densities), the successive curves run into each other or combine so that the current peaks are offset from the actual transitions. This phenomenon, more generally known as peak shift, increase with the frequency of transitions so that, for high recording densities, the peaks can be shifted by up to about one third of the smallest space which can separate two transitions. The decoding circuits must therefore be very active, the more so since to this shifting variations in the speed of travel of the recording carrier are added. Various attempts have therefore been made with a view to reducing the size of the peak shift.
Results have been obtained in this direction by using a digital recording signal similar to an analogue recording signal. Experience has in fact shown a reduction in peak shift for high write densities, above around 200 inversions of flux per millimeter, with a composite recording signal formed by the superposition of a high frequency, constant amplitude, magnetic biasing alternating signal on the digital coded data signal.
In this composite recording signal, each transition is represented by a difference in peak amplitudes of the same sign as two neighbouring half-waves of the biasing signal which are present respectively before and after the instant of transition corresponding to the data signal. Thus, the high frequency biasing is of interest from the moment when these two alternations are separated by a fixed time interval, theoretically corresponding to the period of the biasing signal and resulting in a suppression of the peak shift. However, in alternating biasing, digital recording devices of the prior art, this time interval can deviate unequally and erratically from the value of this period and can cause uncertainties and errors in decoding the signal registered by these devices. These deviations result from the distribution of transitions in the coded digital data signal, the latter being therefore able to arise at any instant in a period of the magnetic biasing signal and act so that the superposition of the two signals is more or less favorable. The more favorable situation (zero deviation) occurs when there is a coincidence between a transition of a given sign and the peak amplitude of the same sign of the high frequency biasing signal. On the other hand, the deviation is maximum when the transition of a given sign occurs at the moment when the biasing signal reaches a peak amplitude of the opposite sign, in which case the following peak amplitude is delayed by about half a wave-length of the biasing signal.
It follows that the size of the peak shift depends on the phase of the magnetic biasing signal with respect to the coded signal and that, if the peak shift is on average effectively reduced by the alternating biasing, relatively high values can be obtained for certain transitions and very active circuits will be needed for the reading and decoding of signals recorded in this manner.
To avoid this peak shift, it would appear of interest to render the biasing signal synchronous, as regards frequency, with the clock for controlling the coded digital signal to be recorded. However, because the positive and negative transitions in the coded signal are distributed in a random manner, the phase that exists between each transition and the magnetic biasing signal remains uncertain, so that more or less favorable cases will still occur, as in the preceding case.
It should be apparent, on the other hand, that the increase in the frequency of the biasing signal with respect to the higher frequency of recurrence of transitions diminishes the peak shift effect. Also, the increase in the frequency of the biasing signal is in practice quickly limited by the fact that it raises the electromagnetic losses in the materials forming the recording heads as a result. Furthermore, an attenuation of the peak shift is observed when there is a judicious relationship between the frequency of the magnetic biasing signal and the clock frequency according to the rhythm at which the coding is effected. Nevertheless, there remain unfavorable cases which could still cause alteration of the restored message in certain cases and, as a result, necessitate the presence of a sufficiently improved read and decoding device to remove these risks.